1. This invention relates to the art of plastic bags, and in particular to a method of opening bags of supple polymeric material subject to interlayer cling. The invention also relates to collapsed bags made of supple materials, such as thermoplastic films, which have, integral to their design, features which render them easy to open from the collapsed state so they can be filled with materials. The invention applies to bags of plain or side-gussetted design.
2. Description of Related Art
Collapsed plastic bags as supplied empty and ready for use from stacks, rolls or various kinds of dispensers are often difficult or tedious to open manually, partly due to being packed for a long period in a flat or compressed state and sometimes due as well to factors arising from manufacture methods, for instance “cold welding” by which cut edges tend to form a weak bond, or materials, for instance “static cling”. Such problems affect most severely bags made of thin films. With thick materials, flexing two adhered layers can place tension on the layer outermost in the curve and compression on the layer innermost in the curve and, depending on thickness and elasticity and the radius of flexion, a point may be easily reached where the static force is overcome by the energy stored in the compressed and tensioned layers so that they spontaneously disengage. This process is not reliable with thin-film bags because even with very tight curves the tension and compression differential is small on account of the difference in radius of curvature being small in turn on account of the small layer thickness, and the tension and compression generated is more easily accommodated by elasticity of the material and layers often do not disengage. Thin film bags are used routinely as checkout bags, produce bags, and general purpose bags. It is thus of advantage to design such bags to be more easily opened, in order to save time and reduce wastage of bags that are damaged during the opening attempt or discarded in frustration.
Additives may sometimes help to make layers separable, but additives generally add cost and also add a further control step in the manufacturing. Furthermore, additives may have unwanted effects such as odor or allergenicity, and some plastic additives have derived from animal fats that in some traditions and religions strictly prohibited—for example animal-derived components even as an ingredient in food packaging materials may be unacceptable under Kosher, Halal, Hindu, or strict vegetarian standards. The public has also been sensitised to animal-derived substances used in non-traditional ways by the BSE (Bovine Spongiform Encephalopathy) problem, and also by the recent finding that some plastic additives act as hormone analogues with implications for human health and also ecological effects. Therefore not only is a solution needed to make collapsed bags easier to handle, but that solution is preferable and more consumer-acceptable if it is mechanical rather than chemical.
In confirmation of the above, the number of patents for mechanical methods show that major companies recognise a strong desire for mechanical solutions to ease handling of plastic bags.
CA 916383 (Walsh and Klein assigned to Leco Industries Ltd) teaches a method of selective cooling of a extruded polymeric tube for preparing blown material with thicker and thinner portions in the same sheet. Because increased thickness of material generally reduces the interlayer adhesion problem, this can be used to make bags with thicker material near the mouth while economising on the use of material overall. An added benefit may be relatively increased strength in the area of handles. This approach to improving the opening properties of bags is now public domain and can be seen in the market, including in small bags at retail establishments like Canadian Tire. But if the objective is to have a thickened zone where the bag mouth occurs for greater ease in opening, application of the thick-thin extrusion method is restricted to bags in which the vertical bag axis perpendicular to the direction of extrusion, for example common side-weld bags with punched handles, and not in that sense applicable to bags supplied on rolls, or T-shirt bags, other bags whose vertical axis is parallel to the direction of extrusion.
Block-headed bags, bags with tear tabs at the top, tabs in a stack of bags being block-welded together and provided with a hanger hole, are popular in many department stores. They are commonly based on the T-shirt plan but have a design address ease of handling. Block-headed T-shirt bags are typically used on racks with pegs supporting the bag handles and most critically a block-welded tab of tear-off sections of a wad of bags. These tabs are in the mouth area of each bag and attached by a weakened or partially cut zone to the main portion of the bag. When the bag is removed from the stack the tab remains behind. The principle is that the user can grasp the front of the front-most bag and that the force so applied will gather the front panel until a small region of that panel separates from the back layer, becomes folded and pinched by the user; then the idea is that pulling on the portion of the front panel so grasped will cause failure of the weakened portion whereby the front panel attaches to the block-welded tab and then further pulling against resistance maintained by the corresponding attachment of the back panel. This renders the bag open and supported by the rack for filling. Removal of the filled bag separates the back panel from the back panel tab, leaving the waste tab, leaving exposed the front panel of the next bag. In practise however, while an improvement over plain T-shirt bags, reliable performance requires moist or sticky fingers, and performance is poor if the bags have to be used without racks (many situations are not compatible with racks, and sometimes supply is short and the wrong size bags are all that are available), which presumably is why many stores still do not use this type of bag.
A further type of mechanical approach to easy opening of produce bags is known in the market (Sealed Air Corp. listing U.S. Pat. Nos. 5,941,393; 5,556,019 on product; and QuikStar™ listing U.S. Pat. No. 5,752,666 on product). It is analogous to a deeply gussetted bag which is then folded on its vertical axis, and which has tabs of a stack of bags block-welded and with a hanger hole. Tabs of each bag are connected by weakened zones to a part of either the front or the back panel (four tabs per bag). This concept gives a multiplicity of layers in which ease of opening benefits from the greater tension-compression differential achieved between innermost and outermost layers subjected to bending around a given radius as formed when the user pinches several layers simultaneously. Ease of opening also benefits from the improved chance of the cling between at least two adjacent layers being weak enough to be easily overcome by simple finger friction.
U.S. Pat. No. 5,611,627 to Belias et al. discloses for flat bags a type having at least one main panel top edge cut on an undulating sinusoidal or like pattern and the other panel the other panel being either flat or similarly shaped edge but cut 180 degrees out of phase. This yields upward projections from the mouth of the bag that can be used to grasp and separate main panels and also to tie the bag closed.
U.S. Pat. No. 3,023,947 to McDuffie confronts problems associated with semirigid multilayer paper bags that are to be opened for filling with bulk materials and then reclosed by sewing on assembly lines. McDuffie's solution uses gusset panels that are both offset and glued to main sidewalls, reforming the bag mouth as a parallelogram and leaving the remaining gussets to form a pair of planar semirigid struts that come under compression when the bag as a whole is placed under horizontal tension in the plane of the main panels; the struts swing and expand the bag mouth, which is a parallelogram because of the glued gusset panel. In McDuffie's solution the bag opening is limited to a width about one-half of the general cross-section of the bag. McDuffie's approach uses not shearing as much as prying, the bag being semirigid; the pulling on McDuffie's tabs causes the free gusset to rotate away from the adjacent main (front or rear) panel, thus prying the front and rear panels apart (vs. peeling of extra folds or shearing of main panels or portions thereof), as in Hoover or the present invention). Indeed: the ratio of direct outward motion per unit lateral motion (pull) in McDuffie's solution can be calculated as [sin A/(1−cos A)], where A represents the angle so formed between the free gusset and adjacent main panel. Initially this ratio is infinity (i.e. all pull and no shear). Just after initiation, assuming an angle of 1 degree, the outward pull is over 100 times as much as the shear. The only shearing that would occur in McDuffie's solution would be better described as a brief rubbing of some portion of the center of the main panels that remain briefly in contact due to air pressure retarding (but not preventing) the opening of the bag. A planar strut as per McDuffie is unlikely to be practicable in thin-film bags. While the reduced opening in McDuffie's solution suffices to accommodate the ingress of granular goods or goods of relatively small dimensions, Hoover and the present invention allow full opening of the bag mouth for items up to the size of the bag volume itself.
U.S. Pat. No. 4,911,560 to Hoover et al (also filed as CA 1,329,384) addresses supple plastic bags and uses an approach that combines shearing of a part of the front and back panels and an unpeeling of extra vertical pleats which are made in the main panels of the bag and arranged in such a way as to collapse or unfold when the is placed under horizontal tension. If the extra pleats are located opposite each other the action is all peeling and no shear, if otherwise there is a combination. To open this type of bag requires a greater amount of movement, and a larger workspace space to accommodate it, to take up the slack provided by unfolding of the extra pleats. This solution creates at least two further problems. In many applications extra folds would be undesirable because of both complication of the printing operation and the subsequent disruption of any printed image that covers the area of the pleats. Also, this approach necessitates extra folds not otherwise required in the bag, so there is added difficulty in manufacture and indeed one source (C. Hutton of East Coast Converters, St. John's) considers it impossible from his experience.
Paper bags and plastic bags may have analogy by virtue of use, but not from the standpoint of practitioners of either art: techniques of man of cutting and gluing paper sheets are unlike those in the business of extruding thermoplastic materials. The former involves already-formed planar feedstock, laminating of multiple layers, cutting into patterned shapes that are perforated, crimped, glued, stapled, or stitched together and requires knowledge of cutters, rollers, stitchers, and glues; whereas the latter involves pelletised feedstock that is mixed, pumped, heated, inflated, cooled, and those ordinarily skilled in the art require familiarity with melting points, extrusion of tubular material and expansion by a controlled bubble of air, frost lines, uniform or structured cooling rates. Training and qualification in the one would be sparse recommendation for employment in the other. Most strikingly, even despite citing McDuffie, Hoover et al did not contemplate the use of offset main panels as an alternative to the additional pleats, and neither is there any evidence that McDuffie's solution influenced any practise in the art of thermoplastic resin bags. In the case of Hoover et al, this implies either that McDuffie did not present itself as a solution, or that Hoover and proprietors chose to patent an inferior solution and rely on McDuffie's solution not being realised by others.
The number of patents addressing mechanical approaches to rendering supple bags easy to open is evidence of a long-felt need and the prospect of commercial returns from solutions to the problem. No convenient solution has so far been found, as is evidenced by the frustration many shoppers feel in the local supermarket.